Using dual isotopes to evaluate sources and transformations of nitrate in the West Lake watershed, eastern China

• The West Lake watershed includes the West Lake which is in the World Heritage List.
• Surface water and groundwater quality were evaluated using statistical approaches.
• Using δ15N and δ18O to detect the nitrate sources at the West Lake watershed is rare.
• A significant degree of nitrification in water at the West Lake watershed was found.
• It is essential to reduce the N in transfer water entering the West Lake.

The West Lake is a World Heritage site in the West Lake watershed in eastern China. In this study, the hydrogeological and dual isotopic approaches were integrated to evaluate the seasonal and spatial variations of nitrate (NO3−) in the West Lake watershed, and to characterize NO3− sources and transformations. The results revealed that the geochemical facies of the water samples were dominated by Ca2 + + Na+–HCO3− + SO42− in the surface water and transfer water, Ca2 + + Na+–HCO3− and Ca2 + + Na+–SO42 − in the groundwater, which most likely reflect natural reactions and anthropogenic inputs. About 13% of the groundwater samples containing NO3− exceeded the World Health Organization (WHO) standard of 10 mg N L− 1. NO3− was the dominant form of total nitrogen (TN) and was the main surface water contaminant in the West Lake watershed. The δ15NNO3 and δ18ONO3 values indicated that the dominant NO3− sources in surface water were soil nitrogen (soil N) and chemical fertilizers, while the main NO3− sources in groundwater were soil N from the forest, chemical fertilizers and manure in the tea garden, domestic sewage from the small, old residential area in the forest as well as urban areas. The distribution of NO3− in groundwater was strongly influenced by land use. Results also suggest that there was significant nitrification in surface water and groundwater in the West Lake watershed, and that there were also denitrification processes in groundwater. The annual net fluxes of TN, NO3−, and NH4+ into the West Lake were 2.0 × 104, 4.0 × 103, and 1.31 × 104 kg as N, respectively.